Java PriorityQueue broken?

I was trying to use PriorityQueue on my class and found mystic errors. So I did this test:

  // create priority queue
  PriorityQueue < Integer >  prq = new PriorityQueue < Integer > () ;

  // insert values in the queue
  for ( int i = 10; i  >  0; i-- ) {
     prq.add (i) ;
  }
  System.out.println("Priority queue values are: " + prq) ;

And the output is weird:

Priority queue values are: [1, 2, 5, 4, 3, 9, 6, 10, 7, 8]

Anybody know what the issues are with PriorityQueue? My Java version:

java version "1.8.0_144" Java(TM) SE Runtime Environment (build 1.8.0_144-b01) Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)